1. Field of the Invention
This invention relates, generally, to electronic devices, and, specifically to a method for making a Josephson junction, from NbN/Si/NbN, at elevated substrate temperatures.
2. Description of the Prior Art
Low resistivity niobium nitride thin films are usually reactively sputtered at temperatures greater than 700.degree. C. This material has a resistivity of less than 80 .mu., .OMEGA.-cm. NbN which is sputtered at ambient temperature has high resistivity, is often under strain or has a columnar structure. These properties are not the most desirable for the deposition of subsequent layers.
U.S. Pat. No. 4,426,268 reports a method for forming high T.sub.c film of niobium nitride without heating the substrate. Introducing carbon into the film during reactive sputtering increases the T.sub.c of a thin film of niobium nitride to near that of bulk niobium nitride.
Josephson junctions fabricated with refractory NbN or Nb superconducting electrodes incorporate artificial barriers such as silicon, germanium, MgO or other suitable material. These barriers are normally deposited at cooled or ambient substrate temperature to assure an amorphous film with uniform coverage. The barrier is then oxidized to plug any "pinholes".
U.S. Pat. No. 4,458,409 relates to an all niobium Josephson junction cf a niobium base electrode, a niobium-oxide tunnel barrier and a niobium counter electrode. A non-continuous layer of gold covers the tunnel barrier oxide layer. The areas not covered by the gold layer are strongly oxidized. The gold becomes superconducting by the proximity effect and allows a tunnel current to flow.
U.S. Pat. No. 4,176,365 discloses a Josephson tunnel junction with niobium superconductive electrodes separated by a hydrogenated amorphous semiconductor material (silicon, germanium and alloys thereof), doped or undoped. This barrier material provides high current density.
U.S. Pat. No. 4,220,959 is for a Josephson junction with niobium nitride superconductive electrodes and a polycrystalline semiconductor tunnelling barrier of silicon, germanium or an alloy thereof, doped or undoped, between the electrodes.
U.S. Pat. No. 4,768,069 pertains to a Josephson junction of niobium nitride on a substrate, an epitaxial layer on the niobium nitride of a pseudo-binary compound of composition 3-97 atomic % MgO/97-3 atomic % CaO and an epitaxial layer of niobium nitride on the pseudo-binary layer. Since the MgO-CaO solid solution have the same lattice spacing as the niobium nitride a good quality top layer of niobium nitride is obtained.
These techniques can limit the substrate temperature of any subsequent layers or post processing temperature. If they are carried out at room temperature, succeeding processing and post-processing temperatures at higher temperatures can result in material migration, contamination and degradation. High quality NbN is deposited at elevated substrate temperatures. It is desirable to find an artificial barrier which can be deposited at substrate temperatures approaching 700.degree. C. and still maintain a stable interface with NbN. Current barrier technology is not compatible with high substrate temperature.